Artificial joint of a prosthesis

ABSTRACT

Prostheses with artificial joints have only a few metallic and ceramic materials whose compatibility with human or animal tissue has been established. These materials, however, cannot be combined with one another in any manner if a friction pairing occurs as a result, for example, in the case of artificial joints. With artificial joints, joint partners are required to have good mechanical properties as well as tribological properties. According to the invention, a sintered material is thus provided which is comprised of zircon oxide with an addition of 0.1 to 40 wt. % aluminum oxide. This sintered material enables an artificial joint of a prosthesis whose other ceramic materials are comprised of aluminum oxide or zircon oxide with the given proportions of aluminum oxide to be assembled with joint partners.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a ceramic sintered material for prostheses orimplants and also to an artificial joint of a prosthesis, in which atleast one joint partner consists of this ceramic material.

2. Description of the Related Art

It is only possible to use a few materials for prostheses withartificial joints on account of the compatibility that is required withhuman or even animal tissue, the so-called biocompatibility, and thehigh level of frictional loading of the joint partners. Of metallicmaterials, titanium-based alloys, for example TiAl6V4, TiAl6Nb7, andcobalt-chromium (CoCrMo) have proved good. Of plastics materials, inparticular polyethylene is used for the sockets of the hip-jointendoprostheses and aluminium oxide and zirconium oxide are the ceramicmaterials that can be used. Whilst prostheses made of merely one of thematerials that have been mentioned do not raise any problems in human oranimal tissue, the materials cannot be combined together one with theother in an arbitrary manner if friction pairings result, as is the casein artificial joints. Moreover, the presence of the body fluid as acorrosive medium is to be taken into account.

In joints, pairings of joint partners of aluminium oxide, as is thecase, for example, in hip-joint endoprotheses, have proved to beparticularly good. Both the ball and the socket are made of the sameceramic material. As is known from the publication “Frettingkorrosion,ein Problem bci Hüftendoprothesen” by Früh, Willmann and Pfaff fErom“Biomaterials”, 18 (1997), No. 12, pages 873 to 876, joint partners ofaluminium oxide and of zirconium oxide cannot be paired together,because wear that cannot be tolerated occurs as a result. Even a pairingof joint partners of zirconium oxide is not possible on account of thewear.

It is however, possible to produce with zirconium-oxide ceramicmaterials work-pieces which, gives comparable dimensions, have acomparatively high level of reliability in particular againstcomparatively high alternating loads. For this reason, for example, itis not expedient to produce the ball-head diameters of hip-jointendoprostheses below 28 mm in diameter out of aluminium oxide. Diametersbelow 28 mm with the required levels of strength and endurance strengthunder alternating repetition of loads can, however, be realized in aproblem-free manner with ceramic materials made from zirconium oxide.The disadvantage, however, lies in the fact that balls ofzirconium-oxide ceramic material cannot be positioned against socketsmade of aluminium oxide and zirconium oxide. It is therefore usual toposition balls of zirconium oxide in sockets of polyethylene, althoughthis causes particle-abrasion of polyethylene and thus results inmedical problems.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to put forward amaterial, and with that possible pairings of material, for prostheseswith joint partners of ceramic materials, whereby new structuralconfigurations with optimum wear characteristics are rendered possible.

According to one aspect of the present invention, there has beenprovided an artificial joint for a prosthesis, comprising (a) a firstjoint partner; and (b) a second joint partner cooperating with the firstjoint partner; wherein at least the first joint partner is made of asintered material consisting essentially of zirconium oxide and 0.1 to40% by weight aluminum oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE diagrammatically shows component parts of a modularlyconstructed hip-joint endoprosthesis made of the material in accordancewith the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The material in accordance with the invention made from zirconium oxidewith an addition of 0.1 to 40% by weight aluminium oxide presents thepossibility of realizing dimensions of the joint partners, for examplethe diameter of ball diameters of the hip-joint endoprostheses, that liebelow 28 mm, in which case the good thermal and mechanical properties ofthe zirconium oxide are combined with time excellent tribologicalproperties of the aluminium oxide.

The material in accordance with the invention renders possible pairingsof materials in the case of artificial joints of prostheses that couldnot be realized before, for example, for reasons of wear.

Particularly good tribological properties, wear resistance, inconjunction with good thermal and mechanical characteristics, inparticular little heating and high levels of endurance strength duringalternating repetition of loads, are offered by a sintered material ofzirconium oxide with an addition of 15 to 25% by weight aluminium oxide.

Stabilization of the zirconium-oxide phases is advantageously achievedif the proportion of the known stabilizers, for example rare earthoxides, alkaline earth oxides, titanium oxide, chromium oxide or hafniumoxide, lies below 10% by weight.

Furthermore, it is advantageous if both joint partners have the samematerial composition. As a result, the sliding partners are identical interms of their abrasion characteristics. The resistance to wearincreases with an increasing proportion of aluminium oxide.

It can, however, be thoroughly advantageous if one of the joint partnershas a higher proportion of zirconium oxide than the other partner. Thiscan be the case, for example, with the balls of the hip-jointendoprotheses in order to be able to utilize to the full theadvantageous material properties of zirconium oxide, in particular withregard to the dimensions of the ball.

The material composition in accordance with the invention also makes itpossible for one joint partner of zirconium oxide with a proportion ofaluminium oxide of more than 5% by weight to be capable of being pairedwith a joint partner of pure aluminium oxide. As a result, the goodtribological properties of the aluminium oxide can be utilized to thegreatest possible extent.

The properties of the material composition can be utilized, in anadvantageous manner, in an artificial hip joint, in which case the ballof the joint consists of the material with the predominant proportion ofzirconium oxide. Since in particular the ball is the joint partner thatis loaded to the greatest extent, with such a material composition thegood tribological properties of the aluminium oxide are combined withthe good thermal and mechanical properties of the zirconium oxide.

On account of the mechanical and thermal material properties of thezirconium oxide, it is possible, in an advantageous manner, to produceballs of hip-joint prostheses that have a diameter which is smaller than28 mm and to pair them with a socket made of an aluminium-oxide ceramicmaterial in order to render possible optimum wear characteristics.

Turning now to the FIGURE, the hip-joint and endoposthesis 1 consists ofa two-part socket 2 which is anchored in the bone with its metallicsocket housing 3 and accommodates a ceramic socket insert 4, in whosehollow sphere-shaped bearing surface 5 the ball 6 can move. The ball 6also consists of ceramic material and has a conical core 7. The shaft 8,made of a bioinert metal, for example of the cobalt alloy CoCrMo, has apeg 9 whose conical form corresponds to the conical form of the bore 7in the ball 6. The peg 9 is anchored in the bore 7 of the ball 6 bymeans of press fit.

In the present exemplifying embodiment, of the joint partners, the ball6 and the socket inert 4, the ball 6 consists of zirconium oxide with aproportion of aluminium oxide of more than 5% by weight. The socketinsert 4 consists of an aluminium-oxide ceramic material of knowncomposition. The ball 6 has a diameter which is smaller than 28 mm. Thematerial composition in accordance with the invention renders possible asubstantially smaller diameter for the ball than is possible with amaterial of pure aluminium oxide. A ball that is of the specifiedmaterial can be assembled, without risk, together with a socket insertof aluminium oxide to form a joint pairing, without the occurrence ofany abrasive and corrosive damage.

What is claimed is:
 1. An artificial joint for a prosthesis, comprising:(a) a first joint partner; and (b) a second joint partner cooperatingwith the first joint partner; wherein at least said first joint partneris made of a sintered material consisting essentially of zirconium oxideand 0.1 to 40% by weight aluminum oxide.
 2. An artificial jointaccording to claim 1, wherein said sintered material of the first jointpartner consists essentially of zirconium oxide and 15 to 25% by weightaluminum oxide.
 3. An artificial joint according to claim 1, whereinsaid first joint further comprises less than 10% by weight of astabilizer for the zirconium oxide phase.
 4. An artificial jointaccording to claim 1, wherein both said first joint partner and saidsecond joint partner are made of said sintered material.
 5. Anartificial joint according to claim 4, wherein one of said first andsecond joint partners comprises a higher proportion of zirconium oxidethan the other of said first and second joint partners.
 6. An artificialjoint according to claim 1, wherein said second joint partner consistsessentially of aluminum oxide and said first joint partner consistsessentially of zirconium oxide with at least 5% by weight of aluminumoxide.
 7. An artificial joint according to claim 1, wherein said jointcomprises an artificial hip joint.
 8. An artificial joint according toclaim 7, wherein said second joint partner comprises a socket.
 9. Anartificial joint according to claim 8, wherein said first joint partnercomprises a ball.
 10. An artificial according to claim 9, wherein saidball is made of sintered material having a higher proportion ofzirconium oxide than the sintered material of said socket.
 11. Anartificial joint according to claim 9, wherein a diameter of said ballis less than 28 mm.
 12. An artificial joint according to claim 9,wherein only said material of said ball contains zirconium oxide.
 13. Anartificial joint according to claim 4, wherein both of said first andsecond joint members are made of sintered material having essentiallythe same composition.
 14. An artificial joint according to claim 1,wherein said cooperation comprises rubbing contact.